This application claims the priority benefit of Taiwan application Ser. No. 89117336, filed Aug. 28, 2000.
1. Field of the Invention
The invention relates in general to a switching power system, and more particularly, to a load-dependent frequency modulation circuit and a method thereof for a switching power system.
2. Description of the Related Art
The typical switching power system has a constant frequency. That is, to deal with the surge load requirement, the system is directly designed to be able to withstand larger load. Such design has to increase the capacitance of the energy converter (such as a transformer, inductor and capacitor), or to increase the operation frequency to carry the surge load. However, the volume is enlarged and the overall efficiency is decreased. Further, other problems such as noise and cost control are caused.
When the opposite condition occurs, that is, when the switching power system is having a light load or no load, a power saving requirement pops up. To satisfy the requirement, the conventional method is to switch the power system to a low operation frequency mode directly to reduce the energy loss.
FIG. 1 shows the control integrated circuit (IC) 100 and the external related circuit used for a conventional switching power system. Being processed, the output of the switching power system is transmitted to the feedback input terminal 130 via the feedback circuit 120. By the feedback process, the control integrated circuit 100 obtains the load condition and adjusts the output energy of the system. When the system load is larger than a predetermined value (adjusted by the designer), the voltage at the pin 110 of the control integrated circuit 100 is switched to 5V causing a higher operation frequency. When the system load is smaller than the predetermined value, the voltage at the pin 110 has high impedance (High13Z) causing a lower operation frequency. This method can only switch between two steps of higher and lower frequencies. Oscillation is easily caused during switching. The requirement of effectively satisfying the surge load and power saving is not met.
Therefore, the conventional technique has the following drawbacks:
1. The conventional technique directly designs the system into a surge load status. That is, the capacitance of the energy converter is enlarged. The system volume is thus increased with a reduced system efficiency. In addition, other problems such as noise and cost control are caused.
2. The switching power system has only two operation frequencies and easily causes oscillation during switching. Further, the requirement to satisfy both surge load and power saving cannot be met effectively.
The invention provides a load-dependent frequency modulation circuit for a switching power system. The modulation circuit has a control integrated circuit, a feedback circuit and a clock control generator. The control integrated circuit includes a clock signal generation terminal and a feedback input terminal. The clock signal generation terminal is to generate a clock signal. The feedback circuit is to process the output of the switching power system. The processed feedback signal is fed back to the feedback input terminal. The clock control generation circuit is electrically coupled to the feedback input terminal to adjust the clock frequency of the clock signal generation terminal according to the variation of the feedback signal.
The above clock control circuit may also comprise a clock control circuit and a clock generation circuit. The clock control circuit may further include a sampling circuit and a frequency adjust circuit. The sampling circuit outputs the sampling result obtained from sampling the feedback signal. The frequency adjust circuit receives the sampling result from the sampling circuit. According to the sampling result, the control signal output to the clock generation circuit is adjusted.
The invention also provides a load-dependent modulation method for a switching power system with a control integrated circuit. The control integrated circuit has a clock signal generation terminal. A feedback signal is obtained from the output of the switching power system. According to the feedback signal, the clock frequency of the clock signal generation circuit is adjusted.
According to the invention, a clock control circuit is used to combine with the feedback circuit in the system, so as to change the clock frequency of the control integrated circuit. Thus, the clock frequency is correlated to the output load, so that the switching power system may automatically adjust the operation frequency according to the variation of the load.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.